Luminescence spectroscopy

Luminescence spectroscopy is a uniquely distinguished spectroscopic technique. It stands out based on its ability to utilize an inherent chemical property of a chemical substance for its spectroscopic determination. This chemical property is called luminescence, which is defined as spontaneous light emission, stimulated by an external energy source.  We have discussed in detail all the interesting facts about luminescence and luminescence spectroscopy in this article. So, for more insightful information, continue reading!

What is luminescence spectroscopy

Luminescence spectroscopy is a type of molecular spectroscopic technique. It is based on luminescence emission. When a chemical substance is irradiated with a light source, the molecules absorb packets of light energy called photons. Consequently, the electrons present in these molecules undergo rotational, vibrational, and electronic energy changes. The electrons are excited to a higher energy level followed by their de-excitation back to the ground state. This de-excitation liberates energy in the form of light, heat, and/or both. The released energy is captured and recorded for qualitative and quantitative analysis of the targeted chemical compounds.

What is the working principle of luminescence spectroscopy

Step I: Light irradiation

• The sample is irradiated with a light source.
• A diverse range of radiation sources are available in luminescence spectroscopy namely,

1. Ultraviolet-visible (UV-Vis) radiations (180-800 nm)
2. X-ray radiations (1-200 nm)
3. Xenon lamp (300-1300 nm)
4. Low-pressure mercury lamp (254 nm, 302nm, 313 nm)
5. Tunable dye lasers

Step II: Selection of excitation wavelength

• A monochromator (filter or a diffraction grating) is placed next to the light source.
• It selects a specific wavelength to pass through the sample at a time.
• The sample is held in a transparent sample cell made of quartz.  

Step III: The irradiated light interacts with sample molecules

• Molecular electrons absorb incident photons and undergo electronic excitation followed by de-excitation.
• The de-excitation of electrons releases energy that is scattered in different directions.

Step IV: Collection of released energy

• The released energy called luminescence is directed onto a second monochromator.
• This monochromator is positioned at 90^° to the scattered radiation.
• It ensures that all the scattered energy is collected and focused onto the detector.

Step V: Detection

• The detector (photomultiplier tubes) records emitted photon energy as an electrical signal and transmits it to the recorder.
• The recorder finally plots the emission intensity versus wavelength of light emitted as a luminescence spectrum.
• The emission radiation is scanned if the excitation wavelength is kept constant and an emission luminescence spectrum is recorded.
• In contrast to that, a specifically emitted wavelength can be recorded by scanning over a range of excitation wavelengths, and an excitation luminescence spectrum is plotted.

Luminometry

The luminescence spectrum allows both qualitative as well as quantitative chemical analysis. The specific wavelength of light absorbed or emitted gives information about the identity of an unknown chemical substance under study. On the other hand, the concentration of this chemical substance can be determined by the intensity of emitted light. This process of measuring light intensity is called luminometry.  

UV-Vis luminometer

The instrument used for luminescence spectroscopy is known as a spectroluminometer. If an ultraviolet-visible (UV-Vis) radiation source such as a deuterium discharge lamp or a tungsten filament lamp is used then the instrument is called a UV-Vis luminometer. The primary difference between a UV-Vis luminometer and a UV-Vis spectrophotometer is that the latter measures the absorption of light by a sample while the former measures light emission.  

Different types of luminescence

The luminescence phenomenon can be divided into three main sub-types:

Fluorescence

Phosphorescence

Chemiluminescence

The different energy levels within which an electron can transition are known as electronic energy levels. Within each electronic energy level, there are further energy divisions called vibrational energy levels, and within each vibrational energy level, there are several rotational energy levels. The electronic states are also classified into singlet and triplet states.

• Singlet state: All the electrons are spin-paired in the singlet state.
• Triplet state: At least one set of electrons spins is unpaired in the triplet state.

The electronic transition from one to another level depends on the amount of energy absorbed for excitation and then the energy released through de-excitation.

Electronic excitation from a vibrational energy level in the ground electronic state to a vibrational level in a higher electronic state makes the electrons relatively unstable. The electrons thus fall to the lowest vibrational energy level of this new electronic state by losing energy in the form of heat. As this process does not involve any photon emission, so it is called non-radiative relaxation. If the electron moves further from this level to another vibrational level of a different electronic level maintaining the same spin state, then it is known as internal conversion.  

The non-radiative relaxation is followed by radiative relaxation in which the electrons are further de-excited to the ground electronic state by emitting photons. As a consequence, the emitted light photons are of lower energy than the absorbed ones because some of the energy is already lost by internal conversion. This process overall is called fluorescence. As per the spin state rule, a singlet-singlet transition takes place in fluorescence. That means no change occurs in the spin state of the electrons.

Contrarily, the electrons change their spin state from triplet to singlet in phosphorescence. It is called intersystem crossing. However, no internal conversion or heat energy loss is involved in phosphorescence. The electrons are readily de-excited from a higher electronic level to the ground electronic state by radiative relaxation i.e., by emitting photons.

Phosphorescence is a slower process and less common as compared to fluorescence. Phosphorescence is usually the after-effect of fluorescence which is an instantaneous process. Both fluorescence and phosphorescence together is called photoluminescence.

All these processes can be further understood by a Jablonski diagram (see the figure below).

Any of the above-discussed luminescence principles can be involved as the interaction mechanism (step III) in luminescence spectroscopy. Thus, fluorescence spectroscopy is a sub-class of luminescence spectroscopy.

The third important type of luminescence i.e., chemiluminescence is a perfectly natural phenomenon. It is the emission of photons by a chemical substance as the result of a chemical reaction. No external excitation source is required for chemiluminescence. If the chemical reaction occurs naturally in living organisms then the photon emission is called bioluminescence.  Fireflies and jellyfish glow due to this bioluminescence.

The luminescence efficiency also called luminosity depends on the extent of the transformation of excitation energy into emitted light. Only the chemical substances that have a sufficient luminescence efficiency can be analyzed via luminescence spectroscopy. Therefore, luminescence spectroscopy is a compound-specific spectroscopic technique. 

Applications of luminescence spectroscopy

• Luminescence spectroscopy allows selective analysis of many different organic and inorganic molecules.
• Impurities and defects present in a chemical compound or structure can be detected using luminescence spectroscopy.
• It has potential applications in food research.

Further spectroscopic applications can be read about here.

You may also like to practice a few numerical problems related to luminescence spectroscopy.

References

1. Creaser, C. S. and J. R. Sodeau (1990). Luminescence Spectroscopy. Perspectives in Modern Chemical Spectroscopy. D. L. Andrews. Berlin, Heidelberg, Springer Berlin Heidelberg: 103-136.

2. Deshpande, S. S. (2001). “Principles and Applications of Luminescence Spectroscopy.” Critical Reviews in Food Science and Nutrition 41(3): 155-224.

3. G.Schulman, S., J. D.Winefordner and I. M. Kolthoff (1993). Molecular luminescence spectroscopy Wiley.

4. Holliday, K. (2017). Luminescence Spectroscopy, Inorganic Condensed Matter Applications. Encyclopedia of Spectroscopy and Spectrometry (Third Edition). J. C. Lindon, G. E. Tranter and D. W. Koppenaal. Oxford, Academic Press: 627-635.

5. Omary, M. A. and H. H. Patterson (2017). Luminescence, Theory. Encyclopedia of Spectroscopy and Spectrometry (Third Edition). J. C. Lindon, G. E. Tranter and D. W. Koppenaal. Oxford, Academic Press: 636-653.